1. Field of the Invention
The present invention relates to an image processor and an image processing method, and more particularly, to image processing for reducing density unevenness caused by a variation in ejection amount among a plurality of nozzles that eject ink.
2. Description of the Related Art
A printing head used in an inkjet type printer may have a variation in ejection amount among a plurality of nozzles due to manufacturing errors, and the like. If there is such a variation in ejection amount, density unevenness is likely to occur in a printed image.
Conventionally, it is known that, as processing for reducing such density unevenness, a head shading technique as disclosed in Japanese Patent Laid-Open No. H10-013674 (1998) is used. The head shading technique is a technique that corrects image data according to information on an ejection amount of each nozzle. This correction enables the number of finally printed ink dots to be increased or decreased to adjust a density in a printed image.
However, even in the case of using the head shading technique as described above, if two or more types of inks are overlapped to represent some color, a so-called color shift may occur, i.e., a color of an area printed with nozzles having a variation in ejection amount may be different from a color that is supposed to be printed. For example, the case of printing a dot with cyan and magenta inks to print a blue image is described. Due to a variation in ejection amount between nozzles, in an area where printing is performed with a nozzle having a larger magenta ink ejection amount than a standard amount, a size of a magenta ink dot is made larger than a standard size. Along with this, by the correction based on the head shading, the number of magenta ink dots printed in this area is made smaller than a standard number. As a result, a cyan dot, and a dot including a blue area and a magenta area around the blue area are mixed. Accordingly, a color in this area is made different from a color supposed to be printed in an area where blue dots formed by cyan and magenta inks for which respective nozzles have no variation in ejection amount are present.